1. Field of the Invention
The present invention relates to a physical quantity measuring apparatus using an optical fiber laser, and more particularly to an apparatus for measuring a physical quantity such as pressure, temperature, strain and etc., by using a beat frequency of longitudinal modes, transverse modes or two polarization modes.
2. Description of the Prior Art
Generally, a physical quantity measuring apparatus is adapted to measure a physical quantity such as pressure, temperature, strain and etc., by using an element variable in property due to the physical quantities. For measuring temperature, for example, thermocouples, thermistors and mercury thermometers have been used. In the case of measuring pressure, there have been used a piezo-resistance effect sensor in which an electric resistance is varied due to a pressure and a piezo-electric pressure sensor. However, such conventional sensors cannot help being affected by electromagnetic interference, because desired information is obtained in the form of an electrical signal. This makes it difficult to use the sensors in areas where electromagnetic noise occurs frequently. Moreover, if a sensor is made of metal, it cannot be used in a material involving an intensive chemical reaction. These problems may be overcome by using optical fiber sensors.
Optical fiber varies in length and refractive index, depending on external perturbations such as temperature, pressure, strain and etc. Accordingly, the phase and the state of the polarization of the light passing through an optical fiber change under a condition that a certain physical quantity is applied. By measuring these changes, the applied physical quantity can be measured.
Generally, fiber optic interferometer system is utilized in measuring the change of physical quantity. Such an interferometric fiber optic sensors mainly consists of a light source, fiber-optic interferometer including sensing unit, and a detection part and signal processing unit. As the light source, a He--Ne laser or a laser diode is generally used. As the fiber-optic interferometer, there are various types, namely, the Fabry-Perot type, the Mach-Zehnder type and the Michelson type.
FIG. 1A shows a conventional Fabry-Perot type interferometer. It consists of a light source 11 and mirrors glued at each end of the optical fiber 14. This Fabry-Perot type system utilizes the periodic transmission characteristics depending on a phase .phi. of a light beam after it travels the resonator. In FIG. 1A, the reference numerals 12 and 15 designate an interference signal sensing unit and a modulator, respectively. When a certain physical quantity is applied to a portion of the optical fiber 14, the length of the optical path of the resonator becomes varied. Due to such a variation, the phase of a light beam becomes varied after the same light beam passes through the resonator for one round trip. As a result, the peak transmission point becomes varied, as shown in FIG. 1B. Measuring this peak transmission point variation is the principle of sensing. Measuring the transmission point variation can be achieved by modulating the optical length of the resonator by a predetermined length, in particular, a length corresponding to a phase .phi. of 2.pi.. Transmission characteristics at every length of modulation can be obtained in an oscilloscope. Generally, a PZT is used to modulate the optical path length of resonator as shown in FIG. 1A. As an electric field is applied to a PZT wound around an optical fiber, the dimension of the PZT becomes varied, thereby inducing the length variation of the optical fiber. In this Fabry-Perot type interferometer, however, it is difficult to read the peak transmission point accurately. To read the peak transmission point accurately, a complex signal processing is required.
FIGS. 2A and 2B show a Mach-Zehnder type and a Michelson type interferometers, respectively. In FIGS. 2A and 2B, the reference numeral 11 designates a light source, 12 a detection and signal processing unit, 13 a mirror, 14 an optical fiber, and 16 a directional coupler. In these cases, the principle of sensing is measuring intensity of the interference signal between the lights traveling each arm of the interferometer. The intensity of interference signal is proportional to 1+cos (.DELTA..phi.) where .DELTA..phi. is the phase difference between light beams passing through each arms, as shown in FIG. 2C. If a certain physical quantity is applied to one arm of the interferometer, the phase of the light beam passing through it changes, which results in a change of the intensity of interference signal. As shown in FIG. 2C, interference signal intensity varies in a sinusoidal manner with respect to .DELTA..phi.. As a result, a complex signal processing is required to obtain the phase change .DELTA..phi. which is proportional to the amount of the applied physical quantity.
There has been also proposed a method of measuring a physical quantity only by using an optical fiber, without any optical fiber interference system. In this case, the physical quantity can be obtained by measuring a variation of a state of polarization of light beam passing through an optical fiber to which the physical quantity is applied. This case also encounters a limitation on accuracy and complex signal processing.